The present invention relates generally to photomultiplier tubes, and, more specifically, to the repair thereof during original manufacture.
A photomultiplier tube (PMT) is an extremely sensitive detector of light and other radiation and has many uses in commercial applications. The PMT is in the form of a glass vacuum tube in which electrons released by radiation striking a photocathode therein are accelerated for greatly amplifying the signal obtained from small quantities of radiation.
The photocathode of the PMT is located at its distal end, with the opposite proximal end thereof having exposed thereat the ends of multiple electrical leads which extend into the evacuated tube. Also disposed inside the tube is an anode which cooperates with the photocathode therein. And, focusing electrodes and many dynodes cooperate therewith. The dynodes are electrodes specifically configured for emission of secondary electrons which multiply the photoelectrons emitted from the photocathode upon bombardment with incident light or radiation.
The various internal components of the PMT are suitably supported therein, on a ceramic substrate for example, with the electrical leads extending through a significant portion of the length of the tube from the respective dynodes and other electrodes through the proximal end of the tube for being connected into the corresponding electrical device in which they may be used.
For example, one application of the PMT is in a positron emission tomography (PET) scanner for medical diagnostic applications such as detecting tumors. Many other applications of PMTs are found in scientific, medical, and commercial uses.
The PMTs are manufactured by first assembling together the internal components thereof and then inserting this subassembly core into the enclosing glass tube and permanently sealed shut by fusing the glass tube to the glass base through which the electrical leads extend. The glass base typically includes a center tube therethrough through which the assembled tube is evacuated to a sufficient level of vacuum, with the tube then being sealed shut.
The so manufactured PMT is then inspected for any defects which would render it inoperable for its intended use. For example, during the initial manufacture of the core elements, the individual dynodes are typically resistance-welded to the ends of the corresponding electrical leads. During the process of resistance welding, fine whiskers of weld material may be formed at the welded joints. These metal whiskers may break during the assembly process into the enclosing glass tube and be liberated therein.
Furthermore, small particles of ceramic substrate or other materials of the PMT core may also be liberated during the manufacturing process. And, even fabric fibers and hair from manufacturing workers may be found inside the evacuated tubes notwithstanding normal precautions made during the manufacturing process for ensuring clean assembly of the various components.
Since the PMT is evacuated and operated at high voltage during use, these various forms of debris particles or contaminants may adversely affect the performance of the PMT or substantially reduce its useful life.
Inspection of the manufactured tubes may uncover relatively large particles that are equal to or greater than 0.2 mm in length which are deemed to be unacceptable for meeting the desired performance and life specifications for the PMT. In particular, the liberation of the weld metal whiskers is a significant concern, and improved welding techniques are being investigated for reducing the occurrence of this one type of particle in the manufactured PMT.
Such metal whiskers represent the majority of undesirable debris particles typically found in the manufacture of PMTs, and the reduction or elimination thereof can substantially reduce the defect rate. Since the glass tube of the PMT is fused shut under vacuum during the manufacturing process, it is impossible to physically remove any undesirable debris therein, and defective PMTs must then be scrapped at a corresponding substantial cost.
In a related development, a laser is used to bombard or blast the debris particles inside the PMT for reducing their size below the 0.2 mm acceptability criteria. However, in order to blast the debris particles, they must be liberated from the internal components of the PMT and positioned or moved to an accessible area within the trajectory of the laser for blasting thereof, without otherwise damaging the internal components of the PMT, or coating the internal surfaces thereof.
Accordingly, it is desired to provide a method and apparatus for marshaling the debris particles in the photomultiplier tube to an accessible site for subsequent blasting thereof.
An apparatus is provided for resiliently mounting a photomultiplier tube. The tube is sequentially tapped to move a debris particle therein. The tube is initially inclined for moving the particle to a staging site. The tube may then be re-oriented horizontally for moving the debris particle from the staging site to a target site for subsequent laser blasting thereof.